SIGMAR1 screened by a GPCR-related classifier regulates endoplasmic reticulum stress in bladder cancer.	BACKGROUND: Bladder cancer (BC) is one of the most common malignancies worldwide. G protein-coupled receptors (GPCRs) are a large family of transmembrane proteins that are increasingly recognised as key players in cancer biology, affecting cell signalling and the tumour microenvironment. The sigma-1 receptor (SIGMAR1), although not a classical GPCR, has similar functions and is associated with the regulation of ER stress. However, its specific role and mechanism in bladder cancer are still unclear. METHOD: The data sets pertaining to batch sequencing, single-cell RNA sequencing (scRNA-seq), immunotherapy response and clinical pathological characteristics were obtained from the public database. Thereafter, multiple algorithms were employed for the screening of GPCRs and immune cells related to the prognosis of BC. A GPCR-tumour microenvironment (TME) classifier was constructed and validated using different queues and multi-omics methods. The key biological pathways between GPCR-TME subgroups were identified through the utilisation of methodologies such as Gene Set Enrichment Analysis (GSEA), Weighted Gene Co-expression Network Analysis (WGCNA), and Tumour Immunophenotype Tracking (TIP). The expression of SIGMAR1 in BC cell lines and tissue samples was validated by western blotting. The Gene Ontology (GO) and GSEA were employed for biological process enrichment analysis. The biological role of SIGMAR1 in BC was investigated through functional experiments and subcutaneous tumour-bearing experiments in nude mice. The relationship between SIGMAR1 and immune cell infiltration was explored using the CIBERSORT method. RESULTS: A total of 15 types of GPCR and 5 types of immune cells were identified and established as a GPCR-TME classifier. Patients in the GPCR-low + TME-high group exhibited the most favourable prognosis, whereas patients in the GPCR-high + TME-low group demonstrated the least favourable prognosis. The scRNA-seq results revealed an increase in GPCR expression in CD8 + T cells, endothelial cells, and NK cells. GPCR-TME was significantly correlated with overall survival (OS) in BC patients and outperformed a range of clinical parameters, making it an independent risk factor affecting the prognosis of BC patients. In comparison to normal tissues, SIGMAR1 was markedly expressed in BC tissues, and was associated with a poor prognosis. Functional experiments demonstrated that SIGMAR1 deficiency impeded the invasive capacity of cancer cells and restrained cellular proliferation. Moreover, in vivo experiments corroborated that SIGMAR1 deficiency curtailed the growth of xenografts in nude mice. Western blotting analysis revealed that SIGMAR1 silencing intensified endoplasmic reticulum (ER) stress in BC cells and promoted cell apoptosis. Additionally, the expression level of SIGMAR1 was correlated with the level of immune cell infiltration and immune-related functions. CONCLUSION: The construction of a BC-related GPCR-TME classifier enabled the effective prediction of the OS of BC patients and the identification of SIGMAR1, a key factor regulating ER stress in BC. The knockout of SIGMAR1 can destroy its protective effect on ER stress, enhance apoptosis of BC cells, and facilitate further investigation of novel treatment strategies for cancer therapy.	bladder